U.S. Pat. No. 4,796,150 issued Jan. 3, 1989 to Dickey, et al. discloses a mechanical-electrical device for protecting telecommunication equipment from power surges which may occur on tip and/or ring conductors of the transmission lines to which the telecommunication equipment is connected. The device disclosed by Dickey, and similar devices of this type, employ a solid state surge protection circuit, one example of which is disclosed in detail in U.S. Pat. No. 4,408,248 issued Oct. 4, 1983 to Bulley et al.
The Bulley circuit provides protection against surges in both the tip and ring conductors of a transmission line by utilizing a plurality of discrete semiconductor devices. Six of these semiconductor devices are diodes which function as rectifiers to permit the passage of current in only one direction. These diodes are referred to as "steering" diodes for that reason. Another of the plurality of discrete semiconductor devices is a gated Silicon Controlled Rectifier (SCR) controlled by a voltage and/or current sensing circuit.
The Dickey device is complex and includes a nonconductive housing with several mechanical parts. Openings are formed in the housing to receive the discrete semiconductors and the metallic plates associated with the discrete semiconductors. These metallic plates act as electrical connectors and heat dissipaters. The device disclosed in Dickey relies upon external metal springs to hold together the semiconductors upon a failure. The discrete semiconductors and the metallic plates must be mounted manually into the housing, one at a time, which adds greatly to the assembly time and overall cost of the protection device, especially when the metal springs are included as described above.
Copending Application Ser. No. 07/917,778 filed Jul. 21, 1992 in the name of Muni Mitchell, Robert Fried, Lou Schilling, and Willem Einthoven, and entitled "Surge Protector Circuit Module and Method for Fabricating Same" (owned by the assignee hereof) teaches a unitary circuit module adapted to be installed as a single unit in a telecommunications protection device. This unitary circuit module overcomes the problems associated with the Dickey type device. It employs a unique lead frame permitting a plurality of unitary circuit modules to be fabricated using mass production techniques. As a result, the cost of each individual module itself is reduced, as is the cost of the construction and assembly of the telecommunication protection device.
In the invention described in the above noted Mitchell, et al. application, a plurality of surge protection circuit modules are simultaneously formed around spaced portions of an extended lead frame, wherein each module has a set of six steering diodes and an ungated thyristor. The extended lead frame includes three spaced parallel leads for each module. These parallel leads have ends which extend out of each of the housings, and all of these ends of all modules are connected by a bridge. During assembly, each set of diodes and their corresponding thyristor is arranged along the extended lead frame in a fixture. By heating, each lead of each module is soldered between a corresponding pair of diodes, and the diodes on each side of the lead frame are soldered to one of two metallic conductor plates. The thyristor of each module is soldered between the two metallic conductor plates. A plurality of semiconductor subassemblies, wherein each semiconductor subassembly has six diodes, a thyristor, three leads, and two metallic conductor plates, are thus formed.
The semiconductor subassemblies thus formed are removed as a unit from the fixture. The semiconductor subassemblies may be encased simultaneously. The bridge is then removed to separate the modules, and the leads are trimmed and bent as required.
Semiconductor subassemblies of this type are commonly sealed in a nonconductive material, such as epoxy. The epoxy is melted and allowed to cure around the semiconductor subassembly prior to the removal of the bridge. It was also contemplated that the semiconductor subassemblies could be separated before encapsulation.
One application for a surge protector of this type is to protect telephone equipment and users thereof from transient high currents which may be present on telecommunication lines due to lightning strikes. The surge protector reacts to a high current surge by switching to a closed state to thereby connect the telecommunication line carrying the surge to ground. It is, therefore, essential that the surge protector not fail in an open state, leaving the equipment and the users thereof unprotected.
In the event of a high current surge, a surge protector can rapidly heat to a high temperature (e.g., 300.degree.). At such a temperature, a molded epoxy case may start to crack. If cracks begin to form in the epoxy, the cracks tend to propagate through the epoxy. As these cracks propagate through the epoxy, the epoxy can chip, even to such an extent that some chips are brutally ejected from the epoxy. If chipping is severe enough, the components of the semiconductor subassembly, namely the diodes, the thyristor, the leads, and the conductor plates, can physically separate. The separation of the semiconductor subassembly components results in a failure of the surge protector in an open state, leaving the telecommunications equipment and the users unprotected.
High temperatures may also cause the components of the semiconductor subassembly themselves to fracture. Fracture of the components of the semiconductor subassembly may also result in an open circuit condition.